Over the years and especially since 2000 when the Selective Availability (SA) feature of the Global Positioning System (GPS) was deactivated, satellite based positioning has become a widely used technique in a variety of application fields. However, its use remains limited in terms of availability, integrity, accuracy and resistance to interference as described in Civil Aviation Authority, “GPS Integrity and Potential Impact on Aviation Safety” 2003 available at http://www.caa.co.uk/docs/33/CAPAP2003—09.pdf. These limitations indicate areas where current GPS receivers have exhibited a lack of robustness. Availability (and continuity) refers to in-view satellites continuously broadcasting signals. Integrity refers to the reliability of the system and of its compliance with specifications, or that signals are as they should be and any anomaly should be identified in time. Accuracy refers to the resolution of the navigation solution, i.e. the precision of the computed position. This depends on both the Dilution Of Precision (DOP), which models the satellites' geometry, and the User Equivalent Range Error (UERE). Interference resistance is an important characteristic since interference events, whether they are intentional (i.e. jamming) or not, could compromise the raw observation measurements (i.e. code and carrier phase measurements). Unintentional sources of interference include harmonics of other frequency bands, non-linearities of amplifiers, and multipath, which causes superposed reflections added to a direct line of sight (or direct path) signal. Jamming can take the form of narrow- or wide-band, constant or pulsed, fixed or sweeping sinusoidal waves. More sophisticated jammers, such as “spoofers”, could also mimic and alter the true GPS signal by broadcasting another at higher power.
With the advent of more recent Global Navigation Satellite Systems (GNSS), including modernized GPS, the Global Orbiting Navigation Satellite System (GLONASS), Galileo and COMPASS systems, new signals, and new types of signals, are now broadcast, or at least should start being transmitted. These signals help resolve the above limitations of GPS. Indeed, higher signal bandwidths will increase the resistance to interference effects by diluting the impact of a narrow band interference over a larger bandwidth as described in Inside GNSS, “Benefits of the New GPS Civil Signal—The L2C study” vol. 18, pp. 42-56, 2006, available at http://www.insidegnss.com/auto/0706%20Benefits.pdf. The new signals should also provide better positioning accuracy and resistance to multipath since the chip period is shorter as described in M. Meurer, S. Erker, S. Thölert, O. Montenbruck, A. Hauschild, and R. B. Langley, “GPS L5 First Light—A Preliminary Analysis of SVN49's Demonstration Signal,” GPS World, pp. p. 49-58, 2009, available at http://www.gpsworld.com/gnss-system/gps-modernization/innovation-15-signal-first-light-8661, thus requiring smaller correlator spacing and a higher sampling rate. Longer codes will increase cross-correlation protection of the signals and their robustness in weak signal environments. The higher number of satellites will increase availability while integrity should be improved through more detailed navigation messages and the deployment of control stations.
The modernization of existing Global Navigation Satellite Systems and the arrival of new systems have diversified to a great extent the range of navigation signals available for civil use. The additional signals address the four traditional weaknesses of the GPS, namely availability, accuracy, integrity and resistance to interferences. This justifies the importance of implementing new robust acquisition and tracking architectures capable of harvesting much more of the potential of the new signals in a compact design.
Currently, the most economical way to produce a navigation receiver is through an Application Specific Integrated Circuit (ASIC), which provides low-cost devices at high volumes. Therefore, hardware resource use of a GNSS channel is still an important consideration, despite the recent trend for pure software receivers or Software Defined Radios (SDR). Indeed, in ASIC designs that are based on signal-specific channels, and in which channels cannot be reconfigured on-the-fly, chances are good that high percentages of the chip will not be used most of the time. Also, populating many dedicated channels drives IC cost up. This is an important consideration going forward, as increasingly blocks of functionality i.e. such as GPS, or GNSS receivers are being implemented as IP cores and are therefore expected to occupy increasingly less of the overall ASIC real estate available.
Indeed, in the case of a totally Software Defined Receiver for GNSS, implemented on a Personal Computer (PC), there may be no issue regarding which of the navigation signals should be tracked. But most commercially available resource-limited receivers are not as flexible and still rely on parallel architectures implemented on dedicated hardware to cope with the large loads of computation required by the multi-channel tracking process.
What is required is the development of a universal design for an acquisition and tracking channel that applies to all currently defined or planned GNSS signals. In other words, with the advent of present satellite navigation systems using standards that do not necessarily use dedicated hardware elements efficiently, and the anticipated introduction of new satellites, and satellite systems using ever increasingly prevalent standards that may not be so easily addressed by dedicated hardware, there is a great need for an efficient architecture capable of addressing these processing needs.